The Eastern Orthodox Church
The Chalcedonian Orthodox Church — the communion of the seven Ecumenical Councils.
The Eastern Orthodox Church is the communion of Christian churches — Greek, Russian, Serbian, Romanian, Bulgarian, Antiochian, and many others — that accept the doctrinal definitions of all seven Ecumenical Councils, from Nicaea (325) to Nicaea II (787).
Eastern Orthodox vs. Oriental Orthodox
The two terms are sometimes confused. The Eastern Orthodox accept the Council of Chalcedon (451) and its Christology of "two natures in one person." The Oriental Orthodox — Coptic, Ethiopian, Eritrean, Syriac, Armenian, and Malankara — do not accept Chalcedon's formula, though they confess the same faith in Christ using different language.
Great East–West Schism
In AD 1054, mutual excommunications between Rome and Constantinople formalized a long-brewing separation between the Latin West and the Greek East. From that date, the Eastern Orthodox Church has stood apart from Roman Catholicism, though the two share the first millennium of Christian history.
Where Eastern Orthodox Christians live today
With roughly 220 million faithful, the Eastern Orthodox are most numerous in Russia, Ukraine, Romania, Greece, Serbia, Bulgaria, and Georgia. In the United States, the Greek Orthodox Archdiocese, the Orthodox Church in America (OCA), and the Antiochian Archdiocese together number roughly a million faithful and are growing rapidly through both immigration and conversion.